This invention relates to apparatus for controlling the scanning path of a rotary transducer as the transducer scans across a movable record medium and, more particularly, to such apparatus wherein the scanning path of the transducer is controlled when the record medium is moved at a faster-than-recording or slower-than-recording speed, such as during a search mode of a previously recorded video tape.
In one type of signal recording apparatus, such as a video tape recorder (VTR) of the so-called helical scan type, a magnetic tape is wrapped helically about a portion of the periphery of a guide drum, and two magnetic recording heads alternately scan parallel tracks across the tape as the tape is moved. Typically, the magnetic heads are disposed at an angular distance of 180.degree.with respect to each other; and the record tracks are skewed with respect to the direction in which the tape is moved. In one type of helical scan VTR, the adjacent parallel record tracks which are recorded by the alternate passes of the magnetic heads are spaced apart from each other by guard bands. In another type of helical scan VTR, the recording density is increased by eliminating such guard bands and recording the parallel record tracks in substantially contiguous form.
In the helical scan VTR wherein contiguous record tracks are recorded, the problem of crosstalk interference due to unwanted signals which are picked up from adjacent record tracks during a reproducing operation is avoided by recording the adjacent record tracks with magnetic heads having different azimuth angles. As is known, if the luminance signal of a composite color video signal is frequency-modulated onto a relatively higher frequency carrier, then, because of the phenomenon of azimuth loss which is directly related to the frequency of the signals, those signals which are recorded in an adjacent track by a magnetic head having a different azimuth angle than the head which is used to reproduce the recorded signals from a given track will be substantially attenuated. Of course, the magnetic playback head should have the same azimuth angle as the head which was used to record the signals in the tracks which are scanned by that playback head. In this manner, crosstalk interference due to adjacent tracks is substantially avoided.
In many uses of a VTR, it is desired to move the tape rapidly therethrough so as to arrive at a preselected portion from which desired signals can be reproduced. Often, that preselected portion is identified only by recognizing the video picture which is reproduced from the VTR. However, if the tape is transported at its normal playback speed, it is appreciated that a significant amount of time is required until the user recognizes that the tape has been transported to its preselected position. Accordingly, there is a definite need to provide a high-speed search mode in a VTR whereby the tape can be transported rapidly until the preselected portion thereof is reached.
Unfortunately, in a typical VTR that is operated at a high-speed playback mode in either the forward or rewind tape direction, i.e., in a VTR having a high-speed search mode, the video picture which is reproduced during this mode generally is so degraded that the user often is unable to recognize when the tape has been transported to its preselected portion. This is because, and as shown in FIG. 3 of the accompanying drawings, when the tape is transported at high speed, the scanning path of a playback head across the tape does not coincide with the parallel record tracks which had been recorded thereon. For example, if the tape is transported at a speed which is nine times the normal playback speed, the scanning path of the playback head will traverse nine previously recorded record tracks. This means that the playback head will scan a segment of one track which had been recorded with the same azimuth angle as that head, followed by a segment of the next adjacent track which is recorded with a different azimuth angle, followed by a segment of the next following track which is recorded with the same azimuth angle, and so on. As a consequence thereof, and because of the recognized phenomenon of azimuth loss, the signals which are reproduced from those segments of the record tracks which had been recorded with a different azimuth angle will be substantially attenuated, effectively to a zero signal level. Consequently, the video picture which is reproduced from these signals will have large horizontal noise bands corresponding to those signals which are reproduced from the segments that had been recorded with a different azimuth angle. These horizontal noise bands seriously degrade the video picture, making it difficult for the user to recognize the displayed video information. A similar problem exists if the tape is transported at a lower-than-normal playback speed wherein the playback head traverses a plurality of different record tracks during its pass. Here too, segments will be scanned from alternate tracks which had been recorded with the same azimuth angle, these segments being interspersed with segments from tracks which had been recorded with the different azimuth angle.
It has been proposed, for example, in copending application Ser. No. 832,368, filed Sept. 12, 1977, now U.S. Pat. No. 4,141,047 and also in copending application Ser. No. 825,861, filed Aug. 18, 1977, and also in U.S. Pat. No. 4,080,636, and other U.S. applications mentioned therein, that tracking errors in the scanning path of a rotary transducer can be corrected if the transducer is supported on an adjustable support assembly, such as a so-called bi-morph leaf assembly. A bi-morph assembly is adapted to deflect in a direction and by an amount depending upon the polarity and amplitude of an electrical signal applied thereacross. Hence, and as described in the aforementioned applications and patent, the bi-morph leaf assembly can be suitably controlled so as to deflect the transducer supported thereon in a direction transversely of its scanning path so that the transducer is adjusted to coincide with the parallel record tracks which are scanned thereby. Although tracking errors are minimized or avoided by use of a suitably controlled bi-morph leaf assembly, it can be appreciated, from FIG. 3 of the accompanying drawings, that the deflection of such a bi-morph leaf assembly to scan only a single track during the high-speed search mode would be beyond the capability of existing apparatus.